1. Field of the Invention
The present invention relates to a toner for use in electrostatic development, in particular, for use in electrophotography, electrostatic printing, electrostatic printing, and the like.
2. Discussion of Background
Generally in electrophotography and electrostatic printing, electrostatic images formed by various methods on an electrostatic image bearing member are developed by an appropriately charged toner to visible toner images. When necessary, such visible toner images are transferred to an image transfer medium and fixed thereto by the application of heat and/or pressure, whereby copies are obtained.
Currently a dry type development method is widely used for developing latent electrostatic images formed on an electrophotographic photoconductor or on an electrostatic recording medium.
In the dry type development method, there is employed toner which comprises a coloring agent such as a dye and/or a pigment, and when necessary, a releasing agent and a charge controlling agent, which are dispersed in the toner. More specifically, such toner is brought into contact with a charging member such as a blade or a roller, thereby charging the toner to a predetermined polarity, and then a thin layer of the toner is formed on a roller. The thus formed thin toner layer is brought to the vicinity of latent electrostatic images or into contact with latent electrostatic images, whereby the latent electrostatic images are developed to visible toner images. This development method is referred to as mono-component development method. In addition to this mono-component development method, a two-component development method is also employed, in which the above-mentioned toner is mixed with a carrier composed of finely-divided magnetic particles to charge the toner to a predetermined polarity and forms the charged toner into a magnetic brush, and latent electrostatic images are developed to visible toner images by the thus formed magnetic brush.
In the mono-component and two-component development methods, it is required that the toner be uniformly charged with an appropriate amount of electric charge.
Recently a toner capable of providing high quality images and hiving high durability is demanded. In order to meet this demand, it is required that the toner have a small particle size and a narrow particle size distribution, and that each toner particle of the toner have a smooth surface, and a charge controlling agent be uniformly dispersed in each toner particle.
More specifically, the particle size of the toner has a significant effect on the resolution, sharpness, and half-tone reproductivity of images. Furthermore, if the particle size distribution is broad, the durability of the toner is degraded because toner particles with particular particle sizes are selectively used in the development. In addition, when the surface of each toner particle is not smooth, toner particles are partially crushed by the stress applied thereto when the toner particles are stirred during the development, so that the toner particles are excessively finely divided. In the two-component development method, when the thus formed finely-divided toner particles adhere to carrier particles of a two-component developer, and the charging performance of the carrier particles is significantly degraded, while in the mono-component development method, when such finely-divided toner particles are fused to adhere to a member for forming a thin toner layer on a roller, non-printed lines are formed in the developed images.
Furthermore, when a charge controlling agent is not uniformly dispersed in each toner particle, the toner particles tend to be deposited on the background of images.
In the case of a conventional toner, which is produced by a conventional method, to be specific, by fusing and/or kneading a resin, a dye or pigment, a charge controlling agent and, if necessary, with the addition of other components, pulverizing the kneaded mixture by a mechanical or air-impaction pulverizer to produce toner particles, and classifying the toner particles, if toner particles with a small particle size and a narrow particle size distribution are to be produced, the production performance and yield thereof are significantly decreased and the production cost is high. Furthermore, the smaller the particle size, the poorer the charging performance of the toner because of the non-uniform dispersion of the charge controlling agent in each toner particle. In addition, in this case, many minute projections are formed on the surface of each toner particle during the pulverizing the toner so that such pulverized toner particles tend to adhere to the carrier particles or to a member for forming a thin toner layer on a roller.
In such a conventional toner, the charge controlling agent which is generally expensive and works only at the surface of each toner particle is also contained within each toner particle. Thus, the conventional toner is costly.
In order to eliminate the above-mentioned problems of the conventional toner, methods of fixing a charge controlling agent to the surface of a resin particle by striking a charge controlling agent into the resin particle by mechanical impact are disclosed, for instance, in Japanese Laid-Open Patent Applications 62-209541, 63-138358, 63-198070, and 63-311264. These methods would be useful to produce toner particles with a uniform chargeability if a finely-divided charge controlling agent with a uniform particle size distribution is employed. However, firstly, in practice, it is extremely difficult to obtain a finely-divided charge controlling agent with a uniform particle size distribution; and secondary, charge controlling agents generally have high melting points, and when toner particles with a charge controlling agent having such a high melting point at the surface of each toner particle are employed, an extremely large amount of thermal energy will be required for performing image fixing.
In order to transfer toner images formed on a latent image bearing material to an image transfer material and to fix the toner images thereto, a pressure-application heating method by use of a heating roller is in general use.
This method is advantageous over other methods in heat transfer efficiency and high speed image fixing performance. However, in this method, a so-called offset phenomenon tends to take place. This is such a phenomenon that a heated roller comes into direct contact with the surface of a fused toner image with the application of pressure thereto, so that part of a fused toner image is transferred to the surface of the heated roller and the partially transferred toner image is again transferred back to an image transfer material, smearing toner images with the re-transferred toner images.
In order to prevent the occurrence of the offset phenomenon, the surface of an image fixing roller is composed of a silicone rubber or a fluorocarbon resin with excellent releasability, and a releasing agent for preventing the offset phenomenon such as a silicone oil is supplied to an image fixing roller during image fixing. This method is very effective for preventing the offset phenomenon, but has the drawbacks that the releasing agent evaporates and gives an offensive odor when heated, and an apparatus for supplying the releasing agent is required. In order to solve these problems, a method of incorporating a resin with releasability into a toner has been proposed in Japanese Patent Publication 52-3304. This method, however, has the shortcoming that in order to obtain a sufficient releasability for use in practice, a large amount of the resin has to be incorporated into the toner, but it is difficult to uniformly disperse the resin in the toner because of the poor compatibility of a binder resin with the releasing agent used in the resin.
In order to solve these problems, methods of providing a releasing agent layer on an outer layer of a toner particle have been proposed, for instance, in Japanese Laid-Open Patent Applications 63-41861, 63-244053, and 63-300245. However, in these methods, since the surface of each toner particle is covered with a low-molecular-weight releasing agent, the toner particle does not retain an appropriate chargeability. The result is that stable development cannot be attained when used for an extended period of time or when there are changes in ambient conditions. Furthermore, the fluidity of the toner particles is significantly degraded so that it is difficult to smoothly supply the toner to a development section, and also it is difficult to conduct image transfer from a latent image bearing material to an image transfer material. In order to obtain sufficient fluidity and image transfer performance for use in practice, a large amount of a fluidity-imparting agent such as finely-divided inorganic particles is required. However, when such a fluidity-imparting agent is deposited on a photoconductor, the agent has adverse effects on developed images, for instance, with the formation of line-shaped non-developed portions in the developed images.
Japanese Laid-Open Patent Application 3-125156 discloses a method of providing a release-agent-containing resin layer on the surface of each toner particle by depositing on each toner particle finely-divided resin particles which contain a releasing agent.
Furthermore, Japanese Laid-Open Patent Applications 3-166551 and 3-180855 disclose methods of depositing finely-divided particles of a releasing agent and finely-divided particles of a resin on the surface of each toner particle, thereby providing a releasing-agent-containing resin layer on the surface of the toner particle.
In any of the above-mentioned methods, the releasing agent is unprotectedly present on the surface of each toner particle so that no solutions have been provided to the problems such as the degradation of the fluidity, transfer performance, and the degradation of the durability of toner, the filming of toner on the surface of a photoconductor, and the spent phenomenon of a carrier by the deposition of toner on the surface of the carrier.